Eric de Montgolfier has led several high profile investigations
French prosecutors have launched a tax evasion investigation based on data stolen by an ex-employee of HSBC's private client bank in Switzerland.
The stolen files have been decrypted and reveal 127,000 accounts belonging to 79,000 people, prosecutors say.
The number of accounts affected is far higher than first thought, with HSBC saying originally it was fewer than 10.
An HSBC spokesman said that the 127,000 claim was "massively overstated" by the French authorities.
However, Eric de Montgolfier, a high-profile prosecutor based in Nice, told reporters that his department had identified 8,231 French account holders, plus tens of thousands more from other countries.
The data theft from HSBC Private Bank (Suisse) was committed three years ago.
The former employee, Herve Falciani, who worked in HSBC's IT division, fled to France while under investigation in Switzerland.
In January 2009, French authorities, acting on a Swiss warrant, seized the data from Mr Falciani's home.
Last December, HSBC said that details of perhaps 10 accounts were stolen. But in March the bank admitted that 24,000 accounts were affected, 15,000 of which were still active and 9,000 of which had closed since the theft.
It is thought that Italian authorities have already asked Mr de Montgolfier, whose investigations have targeted businessman-turned-politician Bernard Tapie and singer Johnny Halliday, for information on its nationals.
The HSBC spokesman said he "simply did not recognise" the figures quoted by Mr de Montgolfier. The Swiss operation did not have that number of accounts, he said, although he declined to say how many there were.
He added that Mr Falciani would not have had access to data outside of the private banking operation in Switzerland.
News of the French probe comes amid a crackdown by US and some European countries on tax evasion through the use of overseas accounts.
In recent years there has been pressure on Switzerland and Liechtenstein to become more transparent about foreign account holders.
Tax authorities in Germany and the UK have paid for stolen data about account holders. And the US is investigating Swiss bank UBS over its tax avoidance practices.